The invention relates to an optical time-domain reflectometer (OTDR) using heterodyne reception for determining the attenuation of an optical fiber by measuring the backscattered portion of light pulses launched into the fiber. The reflectometer comprises a light source whose light is split into a local-oscillator beam and a measuring beam. The measuring beam is pulsed by an acousto-optical modulator (AOM), and is launched into the fiber to be tested with a frequency which is offset from the optical frequency by an acoustic frequency. The local-oscillation beam and the backscattered portion of the measuring beam are both directed to an optical receiver producing an electric output signal. A signal of a frequency corresponding to the acoustic frequency is obtained from the output signal. The time-dependent amplitude of the acoustic frequency output signal is a measure of the length-dependent attenuation of the optical fiber.
Such an arrangement is described in an article by S. Wright et al. entitled "High Dynamic Rouge Coherent Reflectometer For Fault Location in Monomode and Multimode Fibers" (ECOC 83--9.sup.th European Conference on Optical Communication, pages 177 to 180). This arrangement employs an acousto-optical modulator (AOM) only for shifting the optical frequency of the signal emitted by a light source. To split the light emitted by a laser, a beam splitter is used. To couple the light signals into and out of the test fiber a fiber coupler is employed. As a result of this, only a small part of the light energy emitted by the light source reaches the optical receiver via the test fiber. The heterodyne reception of such an OTDR produces a substantial improvement in signal-to-noise ratio (S/N). However, the gain in sensitivity is reduced as a result of substantial losses in the arrangement.